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What I add is dishwashing liquid! The reason oil and water don't mix is because oil is nonpolar, while water is polar. The "like dissolves like rule" explains why table salt (NaCl) can be dissolved in water (both are polar), while oil cannot (it's nonpolar).

What's special about the dishwashing liquid is that it is a surfactant, short for SURFace ACTing AgeNT. This gives it the ability to break the surface tension of water. Also, the molecules in the dish soap have one side that is polar and attracted to water and another that is nonpolar and soluble in oil, allowing both the polar water molecules and the nonpolar oil molecules to create a colloid called an emulsion.

There are different kinds of emulsions. Mayonnaise is an emulsion of vinegar and oil, which wouldn't mix if egg yolks weren't added.Hide